Bearing assembly and manufacturing method

ABSTRACT

A dry bearing ( 10   2 , FIG.  2 ( f )) for a hinge pin or the like comprises a bush  40   2  mounted in a radially supportive housing ( 16 ), the bush being formed by a circumferentially discontinuous tubular form made by rolling a strip of steel-backed, bearing material having a filled polymer infiltrated sintered bronze (ISB) lining about a mandrel (FIG.  32, 2 ( c )). The tubular form is, after mounting and to provide a more uniform inside diameter, burnished by pushing therethrough a burnishing tool ( 50 ), having a cylindrical portion ( 51 ), of larger diameter than the mounted form. The bearing (and method of manufacture) differ from the art in that a minor part of the lining surface is, prior to passage of the burnishing tool and conveniently when still in strip form, compressed in part as depressions (troughs  44   2 ) arrayed over the surface. Such depressions permit burnishing with a less oversize tool to get said uniform inside diameter, thereby requiring less work in driving the tool and with less risk of shearing the lining components and damaging the surrounding housing with radial pressure during burnishing.

This invention relates to bearing arrangements in the form ofcylindrically tubular bushes or cylindrically part tubular shellsmounted against radial expansion and in particular relates to sucharrangements in which the bush or shell comprises a plasticallydeformable and ductile lining material comprising or including a lowfriction polymer-based material and fillers, referred to hereinafter asa filled polymer, that is compressible in the sense of being less thanfully dense before compression. Such a filled polymer based bearingmaterial is, in the following description and claims, referred to as“filled polymer compressible lining”.

The invention is particularly, but not exclusively, concerned with suchbearing arrangements in which the filled polymer compressible lining isdefined by a filled polymer infiltrated into a sponge-like sinteredmetal matrix to form the lining material, which matrix may itself becarried on a backing strip of solid metal, such as steel or bronze orlike material commonly used as a backing material in bearingapplications. It is known to form bearing arrangements in which thesintered metal matrix is bronze and such lining material is hereinreferred to as ‘filled polymer infiltrated sintered bronze’ or ‘FPISB’for short, and as ‘backed filled polymer infiltrated sintered bronze’ or‘BFPISB’ for short, respectively.

Such BFPISB is manufactured and sold by the Applicant with various lowfriction polymer materials and filled with different combinations offillers. For example, the material known as DU has a polymer base ofPTFE with lead filler and is described in patent specification noGB-A-2172296, the material known as DP which has a polymer base of PTFEand zinc and fine bronze instead of lead and is described in patentspecifications no GB-A-2248238, the material known as DP4 which has PTFEwith fillers of calcium fluoride and fibrillated Kevlar (RTM) and isdescribed in patent specification no GB-A-2279998, and the materialknown as DU (B) which has the same lining material and bronze sinter asDU but a bronze backing instead of steel. The above is not an exhaustivelist, and bearing bushes and shells of such bearing material areemployed, for example, in vehicle door hinges to support a rotatablehinge pin and in suspension components to support reciprocating rods,both in rotational and rectilinear motion.

A typical steel backed, filled polymer infiltrated bronze bearingmaterial is formed as a laminar strip manufactured by depositing bronzepowder onto a steel backing, heating the combination to sinter point todevelop a sponge-like bronze matrix layer, depositing a solvent-bornepaste or mush of the low friction polymer and fillers, rolling toinfiltrate it within the interstices of the bronze layer and againheating the combination to dry and sinter the filled polymer so that itcompletely impregnates the bronze sinter matrix. It is a feature of suchmanufacture that the filled polymer not only infiltrates theinterstitial spaces between the partially fused bronze particles(leaving only about 1% porosity) but also forms a relatively thin skinoverlying them. Depending upon the eventual use of the bearingarrangement an amount of filled polymer may be used that causes suchskin to exist in a thickness of 0.010 mm to 0.04 mm. Such structure issometimes described in terms of the polymer skin being an overlay thatis intimately bonded to a substrate provided by the fused metalparticles, the substrate being itself intimately bonded to a backingstrip where appropriate.

In a typical manufacture, tubular bush bearings are formed by deformingor bending a laminar plate-like blank, cut from such strip, around acylindrical mandrel into a longitudinally slit tubular form having theISB lining innermost. The tubular form, which is of coursecircumferentially discontinuous and unstable against radial forces, isthereafter mounted within a radially stronger housing for reception ofhinge pin or like cylindrical object to be borne thereby.

It will be appreciated that in the manufacture of the tubular bush formthere will be variations in dimensions, particularly the inside andoutside diameters thereof as defined by the mandrel diameter and stripthickness, and that furthermore the variations will be exacerbated whenthe tubular form is finally mounted within a separately manufacturedhousing that is itself subject to manufacturing tolerances.

Whereas a bush having a conventional bearing metal lining or homogeneous(incompressible) polymer can be manufactured to have an undersizedinside diameter and have lining surface material removed by a reamingtool or the like to achieve a desired nominal inside diameter, it is notreadily possible to provide inside diameter accuracy for, in particular,an ISB bearing by removing lining material.

To more clearly illustrate the steps involved in production of such aknown bush bearing arrangement having an ISB lining and understandconstraints placed upon achieving dimensional accuracy, reference ismade to FIGS. 1(a) to 1(f), the bush and its manufacture being known inthe art and briefly described here as an aid to understanding theinvention.

Referring to FIG. 1(d), a bearing arrangement 10 is defined by a tubularbush form 12 that has a circumferential discontinuity 14 and is mountedwithin a radially constraining housing 16. Referring also to FIG. 1(a) alaminar strip 18 of steel backed ISB bearing stock, having steel backing19 and ISB lining 20, manufactured as outlined above and with a widthcorresponding substantially to the desired bush length, is pulled from acoil 22 and at a trimming station 24 its edges are trimmed by chamferingto prepare the eventual bush ends 12 ₁ and 12 ₂. The strip is fed to ablanking station 26 at which a predetermined length is a cropped to forma plate or blank 30. The cropped blank is positioned with the ISBsurface 20 adjacent a cylindrical mandrel 32 that has a circular crosssection of predefined diameter. Referring also to FIG. 1(b), a first ram34 clamps the blank to the mandrel and bends the blank about the mandrelinto a U-shape, thereafter a pair of second rams 36 ₁ and 36 ₂ close itaround the mandrel into substantially tubular form before a third ram 38applies pressure to the ends of the strip, the rams in unison pressingthe blank against the mandrel to effect tubular uniformity with themandrel so that the erstwhile opposite ends of the blank meet as acircumferential discontinuity of the tubular form. Referring also toFIG. 1(c), the tubular form, indicated generally at 40, therefore hasits tubular wall, conveniently identified as 42, correspondingsubstantially to the thickness of the BISB 18. The tubular form 40,still held closed on the mandrel is displaced with the mandrel through adie 44 which defines or gauges the outside diameter of the tubular formand, relative to the mandrel, the thickness of the tubular wall 42. Anychanges in wall thickness necessary to permit it to pass through the dieare the result of elongation or drawing of the radially confinedcomponents of the wall which retain their relative thicknesses, althoughthere may be a certain amount of recovery of wall thickness as thematerials leave the die. The mandrel is thereafter withdrawn from thetubular form to leave the bush.

The structure of the wall on an enlarged scale is illustrated in FIG.1(e), illustrating not only backing strip 18 and ISB 20 but also withinthe ISB the sintered bronze matrix 20 ₁, filled polymer 20 ₂ and thepolymer surface layer 20 ₃.

As a consequence of the circumferential discontinuity 14, the bush 40 isrelatively weak against radial forces and it is mounted for use withinan encircling housing 16 to comprise the aforementioned bearingarrangement 10.

As discussed above, manufacturing tolerances in respect of BISB stripthickness and the operations associated with forming of the bush on themandrel and gauging or defining wall thickness prior to removing it fromthe mandrel give a distribution of inside diameter values and, to alesser extent, wall thickness (and thus outside diameter) values eachwithin a range, conveniently called herein a variation range. By way ofexample, in manufacturing a bush of the order of 20.00 mm insidediameter from bearing stock 18 of some 1.5 mm overall thickness(comprising steel backing 19 of 1.2 mm thickness and ISB layer 20 ofsome 0.3 mm thickness), the internal diameter of the tubular form may bedistributed within an industrially reproducible manufacture in avariation range of 0.04 mm or thereabouts, that is, having a maximuminside diameter ID_(MAX) greater than minimum inside diameter ID_(MIN)by a variation ID_(VAR)=ID_(MAX)−ID_(MIN)=approx 0.04 mm, and avariation range of wall thickness of 0.01-0.02 mm.

This degree of dimensional accuracy in inside diameter of the bush perse may be considered marginally acceptable in respect of receiving anelongate member to be borne by the bush, but it is found that as aresult of mounting such bush within the separately manufactured housing,which may itself have a distribution of inside diameters, the effectiveID_(VAR) for the assembled bush may be greater than 0.08 mm.

As also discussed above, the ISB bearing surface precludes reaming of anundersized bush to the desired inside diameter by removal of excesslining material. However, the ISB material is amenable to so-calledburnishing by running through the mounted bush a hard, smooth body ofdiameter greater than the maximum inside diameter of the mounted bush,which body applies radial pressure to the wall by way of the liningsurface and, in a manner analogous to the above-described gauging of theoutside diameter of mandrel-borne bush by a surrounding die, effects areduction in overall wall thickness by drawing or causing the wallcomponents to flow lengthways of the bush within the radiallyinexpansible housing, similarly maintaining their relative thicknesses.

Referring to FIG. 1(f), a fragment of the arrangement of FIG. 1(d) isshown schematically on an enlarged scale along with a fragment of, aburnishing tool 50 which has a body formed from a rod of tool steelhardened and tempered to 58-62 ARC, the rod comprising a slightbi-conical taper (shown greatly exaggerated) of about 1° at end regionsseparated by a cylindrical central portion 51 having a length less thanthe bush, typically 20% of bush length. In respect of burnishing such asmooth bore bush the burnishing tool has a cylindrical diameter thatexceeds ID_(MAX) by about 30-50% of ID_(VAR).

By way of example, bearing arrangements were produced from a samplenumber of bushes formed as described above which exhibited insidediameters varying between 20.00 and 20.05 mm, that is, with an ID_(VAR)of about 0.050 mm. The burnishing tool 50 had a nominal cylindricaldiameter of 20.072 mm (in the range 20.070 to 20.075 mm that representsthe tolerance of tool diameter), that is, exceeding ID_(MAX) by about45% of ID_(VAR). The burnishing tool was pushed through the mounted bushin the direction of arrow 54 and, as illustrated in a schematic way,with the steel backing 19 supported by the housing 16, the cylindricalportion of the tool applied local radial pressure to the BISB 20 whichis compressed between the tool and lining.

Apart from any marginal degree of porosity of the lining material 20, itis essentially incompressible and behaves homogeneously so that passageof the tool requires a thinning of the wall 42 as a whole by flow of allof the components axially; such thinning retains the relativethicknesses of the component layers of the wall and comprises a mixtureof plastic deformation wherein the bush is elongated permanently andelastic deformation whereby the wall regains some of its thickness afterthe tool has passed, principally due to the backing being deformed wellwithin its elastic limits of the steel. The inside diameter afterburnishing is a function of the extent to which the diameter of theburnishing tool exceeds the inside diameter of the unburnished bush andthe recovery (which is itself a function of the degree of compressionthat is related to the excess of the total diameter), but in thisexample, and as seen from the Figure, the after-burnishing insidediameter distribution between ID_(BMIIN) and ID_(BMAX) (=ID_(BVAR)) wasless than before burnishing and typically to the level of variationachieved for the bush manufacture per se eliminating the additionalvariation due to mounting in the housing; typically, for ID_(VAR)=0.050mm, ID_(BVAR)=0.025 mm.

However, there are a number of difficulties and limitations attached toimplementing such burnishing technique. Because of the nature of thelining material it is only possible to effect an increase in the insidediameter of such a mounted bush by reducing the overall thickness of thewall which, as is seen from the above description is manifested as anelongation of the bush by ‘flow’ of the components of the wall inresponse to the radial compression exerted by the burnishing tool.However, the deformation of the bush wall, which comprises largely thesteel backing strip, is governed by the behaviour of the steel which,for a relatively small extension envisioned in this situation, exhibitsa non-negligible, and unpredictable, degree of recovery. That is, thediameter of the burnishing tool has to be sufficiently large in relationto the initial inside diameter that it causes greater deformation of thebush wall than is really required in order to allow for this recovery.This greater deformation not only requires an energy input which islarger than the final change suggests but also that level of initialdeformation/energy input is limited by the level at which theaccompanying longitudinal forces begin to shear the polymer materialfrom the lining. Thus, in respect of applying such burnishing to knownmounted bushes of constant wall thickness, not only is a considerableamount of energy wall deformation required to push the burnishing toolthrough the bush but that the recovery in wall thickness thereafterrepresents a continued uncertainty in respect of final inside diameterand together they limit the extent to which the wall can be deformed toeffect a specific increase in inside diameter.

Furthermore, the radial pressure resulting from such driving force andcompression may, in some cases, be too great for the strength of thehousing which may cease to give support to the bush. As the mounting ofthe bush may have to be effected at its point of use rather than incircumstances given over to bush manufacture, such burnishing of themounted bush may therefore also have to be effected near the point ofeventual use, where conditions are not conducive to such difficulties asmay accompany use of the burnishing tool.

Although the above discussion has concentrated upon tubular bushes andbearing arrangements in which the tubular bush form is retained whenmounted, that is, wherein the bearing is circularly sectioned andsubstantially fully cylindrical, it will be appreciated that suchmounted bush bearings may be divided longitudinally into discretearcuate shells which are mounted for radial support and used singly oras a pair, that is, a circularly sectioned, but only part-cylindricalbearing. The inside diameter dimension of a shell or pair of shells maybe established whilst in tubular bush form before division or, for apair of shells, after mounting with respect to each other into abush-like tubular form, so that the technique of burnishing a tubularform mounted in a radially constraining housing may be consideredapplicable to shell-type ISB bearings as for tubular bush ISB bearings.

Also the structures and techniques are applicable to lining materialscomprising the various filled polymers that are compressible andsuitable for such lining use and/or without a solid metal backing.

Such lining structures and burnishing method provide a starting pointfor the present invention, and it is an object of the present inventionto provide a method of manufacturing a circularly sectioned bearinghaving a burnished, radially supported filled polymer compressiblelining with improved dimensional tolerance and ease of manufacture thanhitherto, and a circularly sectioned, burnished tubular bush bearingarrangement and associated burnishing tool for such manufacture.

It is also an object of the present invention to provide a circularlysectioned bush form, having a filled polymer compressible lining,suitable for mounting surrounded by a radially constraining housing andthen having its inside diameter defined by a burnishing tool whenmounted, that permits easier passage of a burnishing tool and exhibitsgreater dimensional accuracy resulting from such passage. It isfurthermore an object of the present invention to provide asubstantially laminar bearing material having a lining filled polymercompressible and suitable for bending to form such a circumferentiallydiscontinuous bush.

According to a first embodiment of the present invention a method ofmaking a circumferentially sectioned bearing having a burnished,radially supported filled polymer compressible lining comprises (i)defining a tubular form, of which the filled polymer compressible liningpresents a bearing surface extending about, and facing inwardly towards,a longitudinal axis, having an internal diameter smaller than thedesired internal diameter of the bearing, (ii) mounting the tubular formin a radially restraining housing to define a mounted form, and (iii)increasing the internal diameter of the mounted form to said desiredinternal diameter by passing therethrough a burnishing tool, having acylindrical portion of length less than the length of the tubular formand diameter in excess of the desired internal diameter of the bearingand operable to effect by said passage compression of the filled polymercompressible lining in a direction substantially perpendicular to thesurface, and is characterised by the step of, prior to passage of a saidburnishing tool through the mounted bush form, effecting at least apartial compression of a minor part of the filled polymer compressiblelining as a plurality of depressions in said bearing surface of themounted tubular form, distributed over the surface.

According to a second embodiment of the present invention a bearing busharrangement comprises (i) a circularly sectioned tubular bush formsurrounding a longitudinal axis and manufactured with an outsidediameter dimensional to locate within a radially outwardly constraininghousing and an internal diameter less than that desired of the bearingbush arrangement said manufactured internal diameter being defined by abearing surface of filled polymer compressible lining facing radiallyinwardly towards said longitudinal axis, and (ii) an associatedburnishing tool, adapted to be passed through the housed tubular formalong said longitudinal axis and having a cylindrical portion ofdiameter in excess of the desired internal diameter of the bearing busharrangement, the arrangement being characterised in that said tubularbush form is manufactured to have, prior to passage of the burnishingtool therethough, said lining partially compressed over a minor part ofthe bearing surface as a plurality of depressions distributed over thesurface.

According to a third embodiment of the present invention a bearingmaterial comprises a substantially laminar, bendable strip having abearing surface defined by a filled polymer compressible lining andwherein a minor part of the lining is partially compressed as aplurality of depressions distributed over the surface.

Embodiments of the invention will now be described by way of examplewith reference to the accompanying drawings, in which

FIG. (1 a) described above is a schematic side view of a blank cuttingstation of a known arrangement for forming bearing bushes, for derivingfrom a continuous strip of steel backed ISB stock a succession ofrectangular, substantially flat blanks,

FIG. 1(b) is a schematic end view of a bending station associated withthe arrangement of FIG. 1(a) illustrating how each blank is bent arounda mandrel and pressed radially into a circumferentially discontinuoustubular bush form,

FIG. 1(c) is a schematic sectional elevation through the mandrel andtubular form of

FIG. (1 b) illustrating how the tubular form is removed from the mandrelby way of a die to define the outside and inside diameters of thetubular bush,

FIG. (1 d) is a sectional elevation through a bearing arrangementcomprising the tubular bush form of FIG. 1(c) mounted enclosed within aradially constraining housing,

FIG. (1 e) is a schematic cross sectional view through the wall of thebush form of FIG. 1(d) illustrating the components of the wall andrelative thicknesses of the components,

FIG. 1(f) is a schematic representation of fragment of the arrangementof FIG. 1 (d) on an enlarged scale illustrating the passage of aburnishing tool therethrough,

FIG. 2(a) is a detail of a cutting station similar to that of FIG. 1(a)but modified in accordance with the present invention to emboss an arrayof depressions in the form of a series of parallel troughs in the ISBlining simultaneously with cropping of the blank from the continuousstrip,

FIG. 2(b) is an end view, similar to FIG. 1(b), but of such an embossedblank in relation to the mandrel and (in part) bent into a tubular form,such that the troughs run between opposite ends of the tubular form,

FIG. 2(c) is an end view, similar to FIG. 2(b) but in which the troughsrun orthogonally and between the edges of the blank that form thecircumferential discontinuity,

FIG. 2(d) is a sectional elevation similar to FIG. 1(c) but showing thetubular bush form of FIG. 2(c),

FIG. 2(e) is a sectional elevation through a bush bearing arrangementcomprising the tubular bush form of FIG. 2(b) enclosed within a radiallyconstraining housing, illustrating the trough depressions which extendlongitudinally of the bush bearing,

FIG. 2(f) is a sectional elevation similar to that of FIG. 2(e) butshowing an alternative bush form, having a radially extending flange andpressed troughs in accordance with FIG. 2(c) which extend transverselyto the longitudinal direction,

FIG. 2(g) is a greatly enlarged section through a fragment of the wallof the bush form of FIG. 2(f) prior to burnishing illustrating the ISBstructure in the vicinity of a trough depression,

FIG. 2(h) is a fragment of the arrangement of FIG. 2(f) on an enlargedscale illustrating the passage of a burnishing tool therethrough,

FIG. 2(i) is an end view, similar to FIG. (a) in which an unembossedblank is shown in part bent into tubular form about a mandrel havingraised projections arrayed about its periphery press said troughs in thelining surface as the blank is bent into tubular form,

FIG. 3(a) is a cross section through a depressing tool for formingelongate troughs in a mounted bush form in a further method inaccordance with the present invention,

FIG. 3(b) is a sectional elevation through a part of the depressing toolof FIG. 3(a) illustrating also its passage through a mounted tubularbush form and the formation of elongate troughs in the lining thereby,and

FIG. 4 shows in sectional elevation a modification of the method anddepressing tool of FIG. 3(b), namely incorporation of the depressingtool with a flanging tool for forming a flange at one end of a tubularbush form that is mounted for radial support over part only of itslength and adaptation of the guide spigot of such flanging tool as adepressing tool thereof to effect elongate troughs in the surface of thebush form by passage of the spigot.

Referring now to FIGS. 2(a) and 2(b), in accordance with the presentinvention the method of manufacturing a bush bearing is generallysimilar to that described above and where it is the same identicalreference numbers are used. The blank forming apparatus uncoils and edgetrims the steel backed ISB 18 but in blanking station 26′ the length ofstrip comprising the blank plate is not only cropped from the strip by astamping action but is also compressed between a flat supporting surface60 adjacent the backing 19 and a profiled embossment surface 62 adjacentthe filled polymer compressable lining material (ISB) 20, whereby anarray of depressions are indented in the lining surface. In forming thedepressions the embossment surface compresses the lining material,initially eliminating any porosity within the filled polymer beforeeffecting a compression or consolidation of the sinter matrix,displacing polymer therefrom in to adjacent regions of the lining andleaving polymer skin or overlay of slightly increased thickness. Thedepressions are formed as a one-dimensional array of elongate troughs 64₁ each extending across the surface of the blank from edge to edge. Thetroughs are uniformly spaced and each has a width no greater than thelining surface between troughs, (conveniently referred to as theinter-depression surface), conveniently having a width about 66% of theinter-depression distance. The depth profile of each trough is open tovariation, but it is essential that any ‘comers’ of the profiled liningsurface created in forming the depressions are radiused to an extentthat inhibits the formation of shear stress concentrations in the liningduring subsequent operations.

The depths and other dimensional relationships of the trough depressionsare open to variation and are discussed hereinafter. However, the troughdepressions are preferably formed to a depth less than 25% of the ISBlining thickness to avoid the above-discussed detrimental effects on itsperformance that result from over compression of the ISB lining, theactual trough depth being chosen with regard to the variations in insidediameter of the bush due to manufacture and mounting and to the requiredinside diameter of the bush bearing in relation to the nominal insidediameter of the bush form per se.

Referring particularly to FIG. 2(b) the profiled cropped blank,indicated at 30 ₁ in the left hand part of the Figure, is bent aroundand pressed against mandrel 32 into a tubular form, as shown in theright hand half of the Figure, by the same ram arrangement describedwith reference to FIG. 1(b), from which it will be seen that the troughsextend axially along the uneven wall of the tubular bush form. Referringalso to FIG. 2(c), it will be seen that the depressions may be formed inblank 30 ₂ at the cropping station 26′ as troughs 64 ₂ running from edgeto edge along the length of the blank so that when the blank is bentaround mandrel 32 into bush form 40 ₂ the troughs extend between theadjacent edges that form the circumferential discontinuity.

Referring also to FIG. 2(d), which for ease of understanding shows thetubular from 40 ₂ supported on the mandrel, the supported tubular formis then passed by way of the die 44 which gauges the outside diameter by(if necessary) stretching the components of the wall axially such thatthey reduce their relative thicknesses when fully enclosed between thedie and mandrel; however, at the locations of the trough depressionswhere the lining material is spaced from the mandrel and the wall is notfully confined by the surfaces of the mandrel and die, irrespective ofthe extent to which the homogeneous backing metal is elongated bypassage through the die the effect is that the overall wall thickness isreduced and the effective depth of each trough depression is reduced asthe interdepression wall thickness is reduced.

Therefore, for a particular batch of tubular bushes manufactured fromthe same strip of material by the same apparatus, a sample number ofbushes so formed may be examined and measured as to their outsidediameters and wall thicknesses, and the extent to which the overall wallthickness and any depression in it is likely to be adjusted by the diemay be ascertained and taken into consideration when determining thedepth to which the trough depressions are initially pressed in the blankin order to produce a gauged bush form that has trough depressions ofthe desired depth in relation to the wall thickness thereof.

FIG. 2(e) shows the bush 40 ₁ mounted in a housing 16 to comprisebearing arrangement 10 ₁ with the trough depressions 64 ₁ extendingaxially of the bush, and FIG. 2(f) shows a similar sectional elevationthrough bearing arrangement 10 ₂ formed by mounting in housing 16 thebush 40 ₂ with the trough depressions 64 ₂ extending circumferentiallyaround the internal surface of the bush. FIG. 2(f) also illustrates acommon variant of having a radially extending flange 66 defined byoutwardly deforming one end of the tubular bush form (and also referredto hereinafter). Conveniently, the region of the blank destined to formthe flange does not have the depressions formed in the surface thereof.

FIG. 2(g) illustrates schematically a fragment of the side wall 42 ₂ ofthe bush 40 ₂ greatly enlarged and illustrating the nature of theprofiled ISB lining 20, that is, the sponge-like bronze sinter 20 ₁,filled polymer 20 ₂ infiltrating the interstices between the sinteredbronze particles and forming a thin low-friction skin or overlay 20 ₃ atthe surface and depression 64 ₂. In particular the Figure illustratesthat as a consequence of pressing the depressions, the lining, that isthe bronze sinter underlying each depression and the polymer skin iscompressed and consolidated with a reduced volume of intersticesinfiltrated with the filled polymer, but, as mentioned above care istaken not to compress it by more than about 25% of its initialthickness.

The orientation of trough depressions 64 ₂ within the tubular bush formof FIGS. 2(d) and 2(f) is particularly suited for describing a finalburnishing operation with reference to FIG. 2(h). The burnishing issimilar to that described with reference to FIG. 1(f), and correspondingreference numerals are used, except for the presence at the internalbush surface of the trough depressions 64 ₂ and a burnishing tool 50 ₁has a cylindrical part 51 ₁ of slightly smaller cylindrical diameterthan burnishing tool 50.

For a bush having manufactured internal diameter of nominally 20.00 mm,and thus a distribution in mounted internal diameter in a range ID_(MIN)to ID_(MAX) (=ID_(VAR)) of 0.050 mm to 0.080 mm that is, 0.050mm<ID_(VAR)<0.080 mm, the trough depressions 64 ₂ are formed in theblank 30 ₂ so that they have a depth of between 0.030 mm and 0.050 mm inthe bush form after any reduction in wall thickness due to gauging theoutside diameter in the die, but in any event deviating from this to anamount less than would give rise to the maximum 25% reduction in liningthickness that is considered detrimental to lining behaviour, asconsidered above.

The burnishing tool 50 ₁ is formed with the diameter of the cylindricalpart 51 ₁ in excess of ID_(MAX) by an amount of 10-30% of ID_(VAR),compared with an excess over ID_(MAX) of 30-50% of ID_(VAR) for theknown smooth bore bush. The burnishing tool is driven through themounted bush form in the usual manner; the effect of the radiallycompressive force acting by way of the inter-depression lining materialand over a relatively short length of bush is to distort the bulk of theporous sinter matrix and filled polymer such that material displacedradially by the burnishing tool body is accommodated in part by fillingthe adjacent trough depression overlain by the burnishing tool body, andonly after such trough depression is filled and the bush becomeseffectively smooth bore again, does continued radial compression resultin the conventional burnishing and any elongation or drawing of thewhole bush wall, that is, proportional thinning of the lining andbacking with its increased energy input demand.

It will be seen that by choosing a burnishing tool having a cylindricaldiameter, in excess of ID_(MAX), that is not significantly greater thanthe product of the trough depression depth and proportion of liningsurface that is depressed, then the volume of inter-depression materialdisplaced should equal the volume of depression left available by thetool without having to deform the backing strip, and so that afterpassage of the tool the lining surface is smooth with no vestiges of thedepressions. However, it will be appreciated that for bushes on whichthe internal diameter is less that ID_(MAX), and this should be nearlyall of them, the diameter of the burnishing tool will exceed that whichexactly fills the depressions and an overall reduction in wall thicknessby drawing of component parts will be necessary, but to a significantlylesser extent than for a smooth bore bush with the same distribution ofinside diameters and requiring a lower total energy input to push theburnishing tool through the bush. Notwithstanding the varying energyrequirements which may exist from bush to bush, in each case burnishinga particular bush requires less energy that if it were formed with asmooth bore. Furthermore because there is less overall compression ofthe whole wall, with correspondingly less recovery and less need toimpart an initially high deflection to compensate for such recovery, notonly does it permit a smaller diameter (less oversized) burnishing tooland less effort to drive it through the bush, the reduced extent of wallthickness recovery is more predictable and means that the final wallthickness can be finished with improved accuracy for a particularburnishing tool diameter. Thus the variation in internal diameters ofthe burnished bush ID_(BVAR) can be improved, not only to the extentthat the tolerances of the housing are eliminated but also reduced belowthe variations found in a newly manufactured bush before mounting in ahousing.

By way of example, for a sample number of bearing arrangements made fromthe bushes having inside diameters in the range 20.00 to 20.05 mm andtrough depressions pressed individually to a depth of 0.050 mm, theburnishing tool 50 ₁ had a diameter of its cylindrical part of 20.05 mm(that is, exceeding ID_(MAX) by 20% of ID_(VAR)) and produced variationsin the inside diameters in the range 20.045 to 20.055 mm, that is,ID_(BVAR) of 0.01 mm.

The fact that the inside diameter of the bush can be defined in partwithout radial compressive displacement of the lining material and thento the extent necessary to compensate for significant recovery of wallthickness, makes it possible, by burnishing, to increase the internaldiameter of the bush to a greater extent than for a smooth bore bush bya suitably dimensioned burnishing tool. Viewed simply, when the insidediameter of the bush bearing is to be increased to a particular size inexcess of that normally achievable with a smooth bore bush (when theenergy input required exceeds a level at which the filled polymer beginsto shear) the correspondingly larger burnishing tool effects the initialincrease in bore, that is, reduction in overall wall thickness, by theredistribution of lining material with little energy input is then ableto effect the remaining decrease in overall wall thickness by elongationof the bush to the limit of the aforesaid maximum energy input. It isfound that when the enlargement of the inside diameter is small andlittle deformation of the (relatively elastic) backing strip isrequired, the small amount of wall thickness recovery gives a smalldistribution ID_(BVAR) of inside diameters: as greater reduction in wallthickness is required and recovery becomes more pronounced thedistribution of ID_(BVAR) increases, but tends to peak and then diminishas the percentage wall elongation increases. Therefore it may bepossible not only to increase the inside diameter of a mounted bush to agreater extent by burnishing than has hitherto been possible, but alsothat such greater increases are achievable with little or even negativedetriment to the distribution of diameters.

When simply reducing the distribution in inside diameter values, therelative ease with which the smaller diameter burnishing tool is driventhrough the mounted bush reflects the lower radial forces exerted by wayof the bush wall on the housing and this provide an ability to burnishsuch bushes within a weaker housing than hitherto or at least with lessrisk of housing damage/greater margin of safety.

It will be understood that if it is acceptable to have vestigialdepressions, possibly to serve as breathing conduits or lubricantholding pockets, then the trough depressions may be made deeper and/orwider in relation to inter-depression surface for a particularburnishing tool diameter or, conversely, the burnishing tool may takecylindrical diameter less in excess of ID_(MAX).

It is found that the direction of the trough depressions does notsignificantly affect the force necessary to drive the burnishing toolthrough the bush nor the accuracy of dimensions achieved. To this endtrough depressions may be directed other than along or orthogonal to thelongitudinal axis of the bush, such as being skewed. Furthermore theyneed not be parallel to each other nor of equal dimensions and more thanone set of intersecting troughs may be employed. It is believedpreferable that the troughs extend between edges of the blank/bush boreto avoid regions where the bush wall is of constant thickness (as in theknown smooth bore form) without providing a local region to receivedisplaced material. But this is not considered essential on presentknowledge. For example such troughs may be spaced from the edges of theblank but joined to each other in a serpentine manner. Alternatively,the depressions may be arrayed other than as elongate troughs and bedisposed as a two dimensional array of individually more limiteddepressions.

As indicated at above the depressions may be formed with differentspacings and/or dimensions as appropriate for the size of bush andthickness of IBS material, provided the depressed area is less than thenon-depressed area and there are no sharp comers that may promoteshearing of the lining material after passage of the burnishing tool,and the initially flat surface may be made to have continuously roundedcorrugations as a result of pressing trough like depressions.

It will be appreciated that although the above description hasconcentrated upon a backed infiltrated sintered bronze material it isapplicable to any bush in which the wall has a lining that behaves in ananalogous manner with or without a backing, that is, responds to thecompression of radial burnishing pressure by some plastic thinning ofthe wall with or without relaxation, and has a filled polymer liningsurface and/or structure not amenable to techniques such as reamingwhich remove material.

It will be appreciated that the depressions may be pressed into thebearing lining by other than pressing a substantially flat blank, forexample by passing the stock strip (or blank) through rollers, one ofwhich indents the IBS lining face, or by providing a profiled surface tothe mandrel against which the tubular form is defined, as illustrated inFIG. 2(i) in which the mandrel 32′ has arrayed about its periphery aseries of projections 32′₁ which each project to an extent that isslightly in excess of the pre-determined depth desired for the elongatetroughs and against which the ISB lining face 20 of the strip is pressedin defining the tubular form.

As a alternative to forming the indentations as elongate troughs whilstthe bush material is in the form of a laminar strip and prior to bendinginto tubular form, depressions in the form of elongate troughs may beformed in the bearing surface of the tubular form after it is mounted ina radially supporting housing and prior to burnishing.

Referring to FIGS. 3(a) and 3(b), a depressing tool 100 is shown incross sectional and sectional elevation respectively. The tool issimilar to the above described burnishing tool in having a bi-conicaltapered end region 101, 102 and a relatively short central cylindricalregion 103. The cylindrical region 103 has a diameter that issubstantially the same as the undersized mounted bush form and hasarrayed about its periphery a series of projections 104 which eachproject to an extent that is slightly in excess of the pre-determineddepth desired for the elongate troughs. The projections are rounded, atleast in one axial direction, and conveniently in all directions, suchas part spherical.

In forming a mounted bearing by a method employing this tool the bush110 is formed with plane internal lining walls 111 and in the manner ofFIGS. 1(a) to 1(e).

The depressing tool 100 is then pushed through the mounted bush form inan axial direction, and conveniently reciprocally, such that theprojections 104 compress the lining by the passage effecting a local,possibly aggressive burnishing action, and define elongate troughs 112.As discussed above the lining material may effect a degree of recoveryafter compression by the compressing tool passage and in dependence uponthe behaviour characteristics of the lining the tool projections aredimensioned to provide, after any such recovery, the desired troughdimensions.

The mounted, and now indented, bush form is thereafter subjected toburnishing by the burnishing tool 50 ₁ described above with reference toFIG. 2, having the uniformly cylindrical central portion of less thanconventional diameter.

It will be appreciated that in forming elongate troughs by such adepressing tool, the tool may be moved axially whilst being rotatedabout its longitudinal axis in order to produce helically skewedtroughs, or even oscillated about the axis to produce ‘wavy’ troughs,and it may be subjected to such rotational motion with reciprocal axialmotion to define intersecting troughs.

A modification of the above trough defining depressing tool 100 and thusthe method which employs it, is illustrated in FIG. 4 in which it formspart of a flanging tool 200 for forming a flange at one end of a tubularbush form (similar to the flange 66 of the bush form 40 ₂ of FIG. 2(f).)

The flanging tool 200 is symmetrical about a longitudinal axis 201 andis reciprocable therealong by means of a hydraulic ram (not shown). Thetool has a longitudinally extending guide spigot part 202, dimensionedto pass through a bush form 205 mounted in a die 206 which supports itradially except for an end region 207, and a shoulder part 208 adaptedto splay the upstanding and unsupported region 207 of the bush form anddeform it to lie flat against the die as a radial flange, shown ghostedat 207 ¹. The flanging tool and flange-forming operation are essentiallyconventional except insofar as the spigot part 202 has its diameteridentical with, the above described compressing tool 100 and carriesprojections 210. Thus, depending upon the precise disposition of theprojections along the length of the guide spigot part, the operation offorming the flange is immediately preceded by, or contemporaneous with,the formation of elongate troughs. It will be appreciated that inpassing the compressing tool through the unsupported region 207,deflection of the wall may mitigate the formation of depressions.However, even if such trough depressions are formed thereat, thepressure exerted thereon in forming the flange applies compressiveforces to the whole surface of that region of such magnitude as toeffectively eliminate all traces of the troughs from the flange surface,which may be employed as a thrust bearing.

It will be appreciated that the methods described with reference toFIGS. 3 and 4, and employing a depressing tool passed through thetubular bush form, are applicable in respect of not onlycircumferentially discontinuous bush, that is, one bent from a laminarblank, but also a circumferentially continuous bush in which the lininghas been defined in the tubular form in the first instance.

Also, it is reiterated that the bush form mounted in a housing may,after burnishing be removed and split lengthways to form bearing shells,or the bush mounted in the housing before burnishing may comprisediscrete semi- cylindrical shells.

Whereas the above description has concentrated upon the provision of acircularly sectioned lining bearing having a filled polymer compressiblelining, wherein the lining has an array of compression depressions priorto burnishing, and possibly prior to defining the tubular form, thelaminer bearing material per se, may, as a stock material in strip form,have a minor part of the lining provided with an array of depressions bycompression of the lining.

What is claimed is:
 1. A method of making a circularly sectioned bearinghaving a radially supported filled polymer compressible liningcomprising the steps of: (i) defining a tubular form of which the filledpolymer compressible lining presents a bearing surface extending about,and facing inwardly towards, a longitudinal axis, having an internaldiameter ID_(MAX) smaller than the desired internal diameter of thebearing and having an axial length; (ii) mounting the tubular form in aradially restraining housing to define a mounted form, and (iii)increasing the internal diameter of the mounted form to said desiredinternal diameter by passing therethrough a burnishing tool having acylindrical portion of length less than said axial length of the tubularform and a diameter in excess of the desired internal diameter of thebearing and operable to effect by said passage compression of the filledpolymer compressible lining in a direction substantially perpendicularto the surface, the method being characterised by the steps of, prior tothe passage of said burnishing tool through the mounted bush form,effecting at least a partial compression of a minor part of the area ofthe filled polymer compressible lining as a plurality of depressions insaid bearing surface of the mounted tubular form, the depressions beingdistributed over the surface and having a depth of less than 25% of thethickness of the filled polymer compressible lining.
 2. A method asclaimed in claim 1 characterised by determining the variations ininternal diameter of the bearing due to tolerances in manufacture andmounting of the mounted form, and passing through the mounted formincluding said bearing surface depressions said burnishing tool having acylindrical portion of diameter in excess of the desired internaldiameter of the bearing by a percentage of said tolerance variations inthe range 10% to 30%.
 3. A method as claimed in claim 1 characterised byeffecting said depressions as elongate troughs.
 4. A method as claimedin claim 3 characterised by forming at least some of said elongatetroughs extending to an edge of the tubular form.
 5. A method as claimedin claim 3, characterised by forming said elongate troughs discretefrom, and substantially uniformly spaced from, each other.
 6. A methodas claimed in claim 5 characterised by forming said elongate troughsindividually having a width in the range 40% to 70% of the distancebetween adjacent troughs.
 7. A method as claimed in claim 3characterised by forming said troughs to a depth in the range 0.035 to0.045 mm with respect to the undepressed surface.
 8. A method as claimedin claim 3 characterised by forming said troughs such that they areelongate in a direction substantially along the longitudinal axis of thetubular form.
 9. A method as claimed in claim 8 characterised byeffecting said elongate trough depressions by passing through themounted tubular bush form a cylindrical depressing tool having surfaceprojections corresponding to said depressions.
 10. A method as claimedin claim 1 characterised by forming said depressions to a depth in therange 0.025 to 0.055 mm with respect to the undepressed surface.
 11. Amethod as claimed in claim 1 characterised by defining the tubular formby disposing a predetermined length of laminar bearing strip material,having at least one surface of said filled polymer compressible lining,with said lining surface against a mandrel and bending the strip aboutthe mandrel, and being characterised by effecting said plurality ofdepressions by applying pressure to said filled polymer compressiblelining surface of the strip prior to disposing it against the mandrel.12. A method as claimed in claim 11 characterised by forming saiddepressions by clamping the laminar strip between a profiled embossmentsurface and an unprofiled support surface.
 13. A method as claimed inclaim 12 characterised by effecting said embossment clampingsimultaneously with cropping said predetermined length from the end ofbearing strip material stock.
 14. A method as claimed in claim 1characterised by defining the tubular form by disposing a predeterminedlength of laminar bearing strip material, having at least one surface ofsaid filled polymer compressible lining, with said surface against amandrel and bending the strip about the mandrel, and being characterisedby effecting said plurality of depressions by applying pressure to saidcompressible filled polymer bearing surface against projections of aprofiled surface of the mandrel.
 15. A method as claimed in claim 1characterised by forming said depressions to define by their depths anotional tubular diameter less than the diameter of the burnishing tool.16. A method as claimed in claim 1 characterised by forming saiddepressions such that the total volume of filled polymer compressiblelining displaced thereby is substantially equal to the volume of filledpolymer compressible lining representing the diameter undersize of themounted tubular form prior to said passage of the burnishing tool.
 17. Amethod as claimed in claim 1 characterised by radiussing any cornersassociated with the depressions to mitigate shearing of components ofthe filled polymer compressible lining by passage of the burnishingtool.
 18. A method as claimed in claim 1 characterised by forming thefilled polymer compressible lining of sintered bronze infiltrated withfilled polymer.